landslides and their controls

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Course Title: Earth Science Paper Title: GEOHAZARDS AND DISASTER MANAGEMENT

Landslides and their ControlLandslides and their Control

ByProf. A. Balasubramanian Centre for Advanced Studies in Earth ScienceUniversity of Mysore, India

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Table of Contents

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After attending this lesson, the user would be able to understand the nature and causative factors of landslides, their characteristics, classifications, triggering mechanisms, and effects.

The methods of controlling the effects of landslides, and avoiding their menace are also highlighted.

Objectives

(…Contd)

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Disaster management methods are to be adopted to mitigate the never ending natural hazards.

This lesson is an important topic in disaster management.

Objectives

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Geologists, engineers, and other earth science professionals often rely on the unique and slightly differing definitions of landslides.

This diversity in definitions reflects the complex nature of the disciplines that are associated with studying the landslide as a major phenomena.

Introduction

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Landslide is a general term used to describe the downslope movement of soil, rock, and organic materials under the effects of gravity and also the landform that results from such movement.

Varying classifications of landslides are associated with specific mechanics of slope failure and the properties and characteristics of failure types.

It is a major subject of study in disaster management.

Introduction

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Earth’s natural calamities include Earthquakes, Tsunamis, Volcanic eruptions, Cyclones, Floods, and Landslides.

Whenever such natural hazards occur, there will be a severe loss to life and properties.

Landslides are the regular natural hazards experienced in the hilly and mountainous areas all over the world.

Natural Calamities

(…Contd)

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The occurrences of landslides have been recorded by many countries. Landslide is a general term used to describe the downslope movement of soil, rock, and organic materials under the effects of gravity and also the landform that results from such movement.

Varying classifications of landslides have been proposed in association with specific mechanics of slope failure, properties and characteristics of failure types.

Natural Calamities

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Hence, it is necessary to understand the following aspects of landslides:

Landslides as natural hazards Causes of landslides Classification of landslides Landslides and other hazards Effects and evaluation of landslides

Understanding the landslides

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Hills and mountains are typical geomorphic features.

They are characterized by gentle or steep slopes.

Sometimes they may have cliff like vertical slopes.

They are all called as hill-slopes.

Hill slopes are typical geomorphic conditions.

Hill slopes

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They are unique climatic zones.

They are characterized by thick soil profiles, richness with soil moisture and plant growth.

Hillslopes have a good drainage of rainwater.

The bed rock configurations are also very unique.

Hill slopes

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The Subsurface Conditions of a hilly terrain may have two distinct zones as overburden and the basement rock.

The overburden may be mostly loose soil, may have a thick weathered zone, adequate natural or man-made vegetation, good root penetration, and all are under the direct influence of Climatic variations, especially rainfall.

Hill slopes

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Landslides as natural hazards

Landslides normally occur on hillslopes.

When the slopes are stable there will be no problem.

When the slopes become unstable then they may lead to landslides.

(…Contd)

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Landslides as natural hazards

Landslides occur when the stability of the slope changes from a stable to an unstable condition.

The change in the stability of a slope may happen due to several factors.

These factors may be acting alone or acting together.

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Impacts of landslides

Occurrence of landslides also change the general landscape or the geomorphic conditions.

The topography and drainage get modified due to these hazards.

Landslides can destroy the forests and other resources.

(…Contd)

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Impacts of landslides

Landslides will also damage the man-made structures like roads, railways, tunnels and buildings on the hilly regions.

The study of landslides helps in identifying the weak zones, classifying the hill slopes in to different categories and to minimize the impacts of landslides.

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The Bedrock is related to the basement rocks of the hill. These may be massive or fresh, structurally weak or deformed.

All of these may be under the influence of a tectonic force originating from the deep interior or a nearby huge structure like a dam.

The next factor is the degree of slope of the hills.

Slope and bedrocks

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Slope is the common factor for a landslide. The categories of slope is to be understood first.

The term slope relates to the angle of inclination of the land surface.

It varies depending upon the geomorphic conditions, bedrock, nature of overburden, vegetation and drainage systems.

Slope and bedrocks

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The following are the categories of slopes:

Cliff > 80 degrees Precipitous 50-80 degrees very steep or steep 20-50 degrees moderate slope 6-20 degrees gentle slope 1-6 degrees flat terrain < 1degree.

Categories of slopes

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There is an engineering classification of slopes as stable (or) unstable slopes.

The Hill slopes may be stable or unstable.

The slope instability comes due to the relations between overburden and bedrock.

It also depends on the limiting angles which control the geomorphic process acting on it.

Classification of slopes

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Stable slopes are below the limiting angle for mass movement. They are normally ranging from 38 to 40 degrees.

Slope instability comes due to the geological, geomorphological and hydrological conditions.

These include hill-slope processes, changes in vegetation, landaus practices, human activities, frequency and intensity of precipitation and local seismicity if any.

Classification of slopes

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The forces acting on the slopes are of two fold as Driving Force (or) Resisting Force.

Landslide occurs when the driving force is greater than the resisting force.

All happens under the influence of gravity.

The increase in driving forces (weight of slope) may happen due to adding water or adding structures.

Forces acting on slopes

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The decrease in resisting forces may happened due to decrease in weight at the bottom of slope or over-steepening or by removing the toe of slope or due to decrease the frictional energy or by adding water which may lubricate and reduces the strength of the rock.

Forces acting on slopes

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Landslides are caused due to several factors. Some may be natural, and some may be human induced.

The natural causes of landslides include:

Change in Hydrologic conditions: The groundwater (pore water) pressure may be acting to destabilize the slope.

Natural causes of landslides

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Change in vegetation: There will be loss or absence of vertical vegetative structure, soil nutrients, and soil structure due to forest fires or mining

Removal of surface mass: The erosion of the toe of a slope by running river water or rainwater

Snowmelt: Weakening of a slope through saturation by snow melt, glaciers melting, or due to heavy rains


(…Contd)

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Seismically induced changes: The occurrence of micro-seismic waves or tremors.

This may also include earthquake-induced liquefaction which may be destabilizing the slopes.

Volcanic eruptions- may also cause landslides.


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The geological factors influencing landslides are:

Continuous rainfall, heavy rainfall

Slope beyond the limiting slope

Soil type, thickness of soil-profile

Bulk-density, permeability and moisture

soil flowage - increase in soil moisture

Plasticity index(Atterberg limit=amount of water added to change a soil from plastic to liquid state of movement).

Geological factors

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Role of vegetation Vegetation plays a dominant role in this hazard. It has

both positive and negative impacts on the slopes. The factors influencing are:

Density of vegetation- dense or sparse

Type of vegetation, grass, plants or trees

Canopy and weight

Root penetration and its role in infiltration

Age and life of vegetation

Rejuvenation pattern of forests

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The overland flow of water play a distinctive role. In fact, water plays the major role in the origin of landslides. The factors are:

Heavy rainfall will induce more percolation

Increase in the levels of saturation of subsurface soil horizons will induce forces drainage patterns, stream frequency and density.

When water infiltrates, a slippery face is created above the bed-rocks.

Role of water

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There are certain Morphological causes for landslides. They are:

Tectonic or volcanic uplift

Glacial rebound

Fluvial, wave, or glacial erosion of slope toe or lateral margins

Subterranean erosion (solution, piping)

Morphological causes

(…Contd)

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Deposition loading slope or its crest

Vegetation removal (by fire, drought)

Thawing

Freeze-and-thaw weathering

Shrink-and-swell weathering.

Morphological causes

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Although landslides are primarily associated with mountainous regions, they can also occur in areas of generally low relief. In low-relief areas, landslides occur as

cut-and-fill failures (roadway and building excavations),

river bluff failures,

Slides in Low-lying areas

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lateral spreading landslides,

collapse of mine-waste piles (especially coal), and

a wide variety of slope failures associated with quarries and open-pit mines.

Slides in Low-lying areas

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Landslides are also caused due to human activities. The major human causes include:

Deforestation & Cultivation

Disturbing the drainage patterns through cut and fill operations

Construction, Blasting, Mining and Quarrying

Vibrations from machinery or heavy traffic and movement of heavy vehicles.

Human activities

Human activities

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Earthwork which alters the shape of a slope and impose new loads on the existing slopes

Removal of deep-rooted vegetation

Bushfires for clearing trees.

Human activities

Human activities

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Landslides are classified into various types based on nature of materials involved and type of movement involved.

The type of movement describes the actual internal mechanics of how the landslide mass is displaced like fall, topple, slide, spread, or flow.

Classification of landslides

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Similarly, the material in a landslide mass may be either a rock or a soil (or both).

The soil is described as earth if mainly composed of sand-sized or finer particles and debris if it is composed of coarser fragments.

Each type of movement can be further subdivided according to specific properties and characteristics.


(…Contd)

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Based on the nature of materials involved, landslides are classified into Rockslides, Debris slides, and Earthslides/landslides.

Based on the type of movement, landslides are classified into these types:


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Slides

Although many types of mass movements are included in the general term "landslide" the more restrictive use of the term refers only to mass movements, where there is a distinct zone of weakness that separates the slide material from more stable underlying material.

The two major types of slides are rotational slides and translational slides.

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This is a slide in which the surface of rupture is curved concavely upward and the slide movement is roughly rotational about an axis that is parallel to the ground surface and transverse across the slide.

The displaced mass may, under certain circumstances, move as a relatively coherent mass along the rupture surface with little internal deformation.

Rotational slides

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The rock slump is a rotational slide. It is related to sliding of a mass of weak rock on a cylindrical or ellipsoidal rupture surface which is not structurally-controlled.

There will be little internal deformation. There may have a large main scarp and characteristic back-tilted bench at the head.

They are usually slow events.

Rotational slides

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Because rotational slides occur most frequently in homogeneous materials, they are the most common landslide occurring in “fill” materials.

These are associated with slopes ranging from about 20 to 40 degrees. In soils, the surface of rupture generally has a depth-to-length ratio between 0.3 to 0.1.

Occurrence of Rotational slides

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The velocity of travel or rate of movement are

extremely slow (less than 0.3 meter or 1 foot

every 5 years) to moderately fast (1.5 meters or 5

feet per month) to rapid.

Occurrence of Rotational slides

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These triggered by intense and (or) sustained rainfall or rapid snowmelt. Such events may lead to the saturation of slopes and increased groundwater levels within the mass.

They are also caused due to rapid drops in river water level following flood events, ground-water levels rising as a result of filling reservoirs, or the rise in level of streams, lakes, and rivers.

All these activities can cause erosion at the base of slopes. These types of slides can also be earthquake-induced.

Triggering mechanism of slides

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Rotational slides can be extremely damaging to structures, roads, and lifelines.

They are not usually life-threatening if movement is slow. Structures situated on the moving mass also can be severely damaged as the mass tilts and deforms.

The large volume of material that is displaced is difficult to permanently stabilize. Such failures can dam rivers and lead to severe flooding.

Effects (direct/indirect) of slides

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Instrumental monitoring to detect movement and the rate of movement is a major method to be implemented.

The disrupted drainage pathways should be restored or reengineered to prevent future water buildup in the slide mass.

Mitigation measures

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It is necessary to establish proper grading and engineering of slopes, wherever possible.

This will reduce the hazard considerably.

Construction of retaining walls at the toe may be effective to slow or deflect the moving soil.

Mitigation measures

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Predictability of Rotational slidesPredictability of Rotational slides

The historical slides can be reactivated at any time. So, prediction is necessary.

The distribution of cracks at the tops (heads) of the slopes are good indicators of the initiation of failures.

Mapping of such regions and controlling their triggers is necessary.

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In this type of slide, the landslide mass moves along a roughly planar surface with little rotation or backward tilting.

A block slide is a translational slide in which the moving mass consists of a single unit or a few closely related units that move downslope as a relatively coherent mass.

Translational slides

(…Contd)

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The translational rockslide is the sliding of a mass of rock on a planar rupture surface, or a wedge of two planes with downslope-oriented intersection.

The rupture surface may be stepped. There will be no internal deformation.

The slide head may be separating from stable rock along a deep, vertical tension crack. These are usually extremely rapid events.


(…Contd)

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The material in the slide may range from loose, unconsolidated soils to extensive slabs of rock, or both.

Translational slides commonly fail along geologic discontinuities such as faults, joints, bedding surfaces, or the contact between rock and soil.


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One of the most common types of landslides, occurring all over the world.

They are found globally in all types of environments and conditions.

Their movement is shallower than rotational slides.

Occurrence of Translational slides

(…Contd)

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Occurrence of Translational slides

The surface of rupture has a distance-to-length ratio of less than 0.1 and can range from small (residential lot size) failures to very large, regional landslides that are kilometers wide.

Their movement may initially be slow (5 feet per month or 1.5 meters per month) but many are moderate in velocity (5 feet per day or 1.5 meters per day) to extremely rapid. With increased velocity, the landslide mass of translational failures may disintegrate and develop into a debris flow.

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These are triggered primarily due to intense rainfall, rise in ground water within the slide due to rainfall, snowmelt, flooding, or other inundation of water resulting from irrigation, or leakage from pipes or human-related disturbances such as undercutting.

These types of landslides can also be induced by earthquakes or tremors.

Triggering mechanism and effects

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Translational slides may initially be slow, damaging property and (or) lifelines. In some cases, they can gain speed and become life-threatening.

They also can dam rivers, causing severe flooding.

Triggering mechanism and effects

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These are to be controlled by adequate drainage systems. It is necessary to prevent sliding or, in the case of an existing failure, to prevent a reactivation of the movement.

The other common corrective measures include leveling, proper grading and drainage, and construction of retaining walls.

Mitigation measures

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More sophisticated remedies in rock include anchors, bolts, and dowels, which in all situations are best implemented by professionals.

Translational slides on moderate to steep slopes are very difficult to stabilize permanently.

These slides have high probability of occurring repetitively in areas where they have occurred in the past, including areas subject to frequent strong earthquakes. Widening cracks at the head or toe bulge may be an indicator of imminent failure.

Mitigation measures

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Rock collapse are also sliding events of a rock mass on an irregular rupture surface consisting of a number of randomly-oriented joints.

Rotational Soil Slumps are sliding of a cohesive soil mass on a cylindrical or ellipsoidal rupture surface. They show little internal deformation.

Debris slide is also a sliding of a landmass of granular material on a shallow, planar surface parallel with the ground.

Rock collapse and soil slumps

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Falls

Falls are abrupt movements of masses of geologic materials, such as rocks and boulders, that become detached from steep slopes or cliffs.

Separation occurs along discontinuities such as fractures, joints, and bedding planes, and movement occurs by free-fall, bouncing, and rolling

(…Contd)

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Falls

Falls are strongly influenced by gravity, mechanical weathering, and the presence of interstitial water.

A fall begins with the detachment of soil or rock, or both, from a steep slope along a surface on which little or no shear displacement has occurred.

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Falls are abrupt, downward movements of rock or earth, or both. They get detached from steep slopes or cliffs.

The falling material usually strikes the lower slope at angles less than the angle of the fall, causing a bouncing action.

The falling mass may break on impact, may begin rolling on the steeper slopes, and may continue to move until reaching a flat terrain.

Rockfalls

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These are common, worldwide, on steep or vertical slopes.

They are also seen in coastal areas, and along rocky banks of rivers and streams.

The volume of material in a fall can vary substantially, from individual rocks or clumps of soil to massive blocks thousands of cubic meters in size.

The velocity may be very rapid to extremely rapid.

Rockfalls

(…Contd)

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There may be free-fall, bouncing and rolling action of the detached soil, rock, and boulders.

The rolling velocity depends on slope steepness.

The triggering mechanism may be undercutting of slope by natural processes such as streams and rivers or differential weathering (such as the freeze/thaw cycle), human activities such as excavation during road building and (or) maintenance, and earthquake shaking or due to intense vibration, by traffic.

Rockfalls

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Effects of rockfalls (direct/indirect)

The falling material can be life-threatening. Rockfalls can damage property beneath the fall-line of large rocks.

Boulders can bounce or roll great distances and damage structures or kill people.

Damage to roads and railroads is particularly high: rockfalls can cause deaths in vehicles hit by rocks and can block highways and railroads.

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These are controlled by constructing rock curtains or other slope covers, protective covers over the roadways, and by constructing retaining walls to prevent rolling or bouncing.

The removal of mass in hazardous target areas, removal of rocks or other materials from highways and railroads can be used.

Corrective measures/mitigation of rockfalls

(…Contd)

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Rock bolts or other similar types of anchoring may be used to stabilize the cliffs. Scaling may lessen the hazard.

Warning signs are recommended in hazardous areas for public awareness.

Stopping or parking under hazardous cliffs should be avoided.

Corrective measures/mitigation of rockfalls

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Mapping of hazardous rockfall areas should be done.

Rock-bounce calculations and estimation methods for delineating the perimeter of rockall zones are to be attempted.

These list of information should be widely published.

Predictability

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The indicators of imminent rockfall include terrain with overhanging rock or fractured or jointed rock along steep slopes.

They should be notified.

The areas that are subjected to frequent freeze-thaw cycles may be affected regularly.

Predictability

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A topple is the forward rotation out of a slope of a mass of soil or rock. It happens around a point or axis below the centre of gravity of the displaced mass.

Toppling is sometimes driven by gravity exerted by the weight of material upslope from the displaced mass.

Topples

(…Contd)

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Toppling failures are distinguished by the forward rotation of a unit or units about some pivotal point, below or low in the unit, under the actions of gravity and forces exerted by adjacent units or by fluids in cracks. These are known to occur globally.

Topples

(…Contd)

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They are often prevalent in columnar-jointed volcanic terrain, as well as along stream and river courses where the banks are very steep.

Their velocity of travel are extremely slow to extremely rapid, sometimes accelerating throughout the movement depending on the distance of travel.

Topples

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Triggering mechanism of topples

Topples are sometimes driven by gravity exerted by material located upslope from the displaced mass and sometimes by water or ice occurring in cracks within the mass.

The other reasons are the vibration, undercutting, differential weathering, excavation, or stream erosion.

The effects can be extremely destructive, especially when failure is sudden and (or) the velocity is rapid.

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In rocky terrains, there are many options for the stabilization of topple-prone areas.

One approach is the reinforcement of these slopes include rock bolts and mechanical and other types of anchoring.

Seepage is also a contributing factor to rock instability.

Hence, the drainage should be considered and addressed as a corrective measure.

Corrective measures/mitigation

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Topples are not generally mapped for susceptibility.

But some inventory of their occurrence should be done in certain areas. Monitoring of topple-prone areas is a useful method. The use of tiltmeters is inevitable.

Predictability of topples

(…Contd)

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Tiltmeters are used to record changes in slope inclination near cracks and areas of greatest vertical movements.

Warning systems based on movement measured by tiltmeters could be effective methods of control.

Predictability of topples

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A flow is a spatially continuous movement in which the surfaces of shear are short-lived, closely spaced, and usually not preserved.

The component velocities in the displacing mass of a flow resemble those in a viscous liquid.

Often, there is a gradation of change from slides to flows, depending on the water content, mobility, and evolution of the movement.

Flows

(…Contd)

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There are five basic categories of flows that differ from one another in fundamental ways.

They are debris flows, debris avalanches, earthflows, mudflows and creeps.

Flows

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A debris flow is a form of rapid mass movement in which a combination of loose soil, rock, organic matter, air, and water mobilize as a slurry that flows downslope.

Debris flows include <50% fine-grained mass.

Debris flows are commonly caused by intense surface-water flow, due to heavy precipitation or rapid snowmelt, that erodes and mobilizes loose soil or rock on steep slopes.

Debris flow

(…Contd)

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Debris flows occur around the world and are prevalent in steep gullies and canyons; they can be intensified when occurring on slopes or in gullies that have been denuded of vegetation due to wildfires or forest logging.

They are common in volcanic areas with weak soil.

Debris flow

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Lahars (Volcanic Debris Flows)

The word “lahar” is an Indonesian term. Lahars are also known as volcanic mudflows.

These are flows that originate on the slopes of volcanoes and are a type of debris flow.

A lahar mobilizes the loose accumulations of tephra (the airborne solids erupted from the volcano) and related debris.

(…Contd)

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Lahars (Volcanic Debris Flows)

They are found in nearly all volcanic areas of the world. Lahars can be hundreds of square kilometers or miles in area and can become larger as they gain speed and accumulate debris as they travel downslope.

They can be small in volume and affect limited areas of the volcano and then dissipate downslope.

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Debris avalanche

Debris avalanches are essentially large, extremely rapid, often open-slope flows formed when an unstable slope collapses and the resulting fragmented debris is rapidly transported away from the slope.

In some cases, snow and ice will contribute to the movement if sufficient water is present, and the flow may become a debris flow and (or) a lahar.

(…Contd)

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Debris avalanche

They occur worldwide in steep terrain environments.

They are also common on very steep volcanoes where they may follow drainage courses.

There are large avalanches that have been known to transport material blocks as large as 3 kilometers in size, several kilometers from their source.

(…Contd)

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Debris avalanche

Their velocity of travel may be rapid to extremely rapid.

Debris avalanches can travel close to 100 meters/sec.

In general, the two types of debris avalanches are those that are “cold” and those that are “hot.”

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The debris avalanches may travel several kilometres before stopping, or they may transform into more water-rich lahars or debris flows that travel many tens of kilometres farther downstream.

Such failures may inundate towns and villages and impair stream quality.

Effects (direct/indirect) of Debris avalanches

(…Contd)

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They move very fast and thus may prove deadly because there is little chance for warning and response.

It is necessary to avoid constructions in the valleys on volcanoes or steep mountain slopes.

Real-time warning systems may help in lessening the damages.


(…Contd)

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However, warning systems may prove difficult due to the speed at which debris avalanches occur—there may not be enough time after the initiation of the event for people to evacuate.

Debris avalanches cannot be stopped or prevented by engineering means because the associated triggering mechanisms are not preventable.


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Earthflows have a characteristic "hourglass" shape.

The slope material liquefies and runs out, forming a bowl or depression at the head.

The flow itself is elongate and usually occurs in fine-grained materials or clay-bearing rocks on moderate slopes and under saturated conditions.

However, dry flows of granular material are also possible.

Earthflows

(…Contd)

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Earthflows occur worldwide in regions underlain by fine-grained soil or very weathered bedrock.

The flows can range from small events of 100 square meters in size to large events encompassing several square kilometres in area.

Their velocity of travel may be slow to very rapid.

Earthflows

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The triggers include saturation of soil due to prolonged or intense rainfall or snowmelt, sudden lowering of adjacent water surfaces causing rapid drawdown of the ground-water table, stream erosion at the bottom of a slope, excavation and construction activities, excessive loading on a slope, earthquakes, or human-induced vibration

Triggers and effects of earthflows

(…Contd)

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The effects may be rapid.

They may run out for considerable distances, potentially resulting in human fatalities, destruction of buildings and linear infrastructure, and damming of rivers with resultant flooding upstream and water siltation problems downstream.


(…Contd)

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Slower earthflows may damage properties and sever linear infrastructure.

Grading of slopes and protecting the base of the slope from erosion or excavation is needed.


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A mudflow is an earthflow consisting of material that is wet enough to flow rapidly and that contains at least 50 percent sand-, silt-, and clay-sized particles.

In some instances, for example in many newspaper reports, mudflows and debris flows are commonly referred to as "mudslides."

Mudflow

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Creep is the informal name for a slow earthflow and consists of the imperceptibly slow, steady downward movement of slope-forming soil or rock.

Movement is caused by internal shear stress sufficient to cause deformation but insufficient to cause failure.

Generally, the three types of creep are:

Creep

(…Contd)

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1. seasonal, where movement is within the depth of soil affected by seasonal changes in soil moisture and temperature;

2. continuous, where shear stress continuously exceeds the strength of the material; and

3. progressive, where slopes are reaching the point of failure for other types of mass movements.

Creep

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Spreads

Spreads are an extension of a cohesive soil or rock mass movement combined with the general subsidence of the fractured mass of cohesive material into softer underlying material.

The spreads may result from liquefaction or flow (and extrusion) of the softer underlying material.

The types of spreads include block spreads, liquefaction spreads, and lateral spreads.

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Lateral spreads usually occur on very gentle slopes or essentially flat terrain, especially where a stronger upper layer of rock or soil undergoes extension and moves above an underlying softer, weaker layer.

Such failures are normally accompanied by some general subsidence into the weaker underlying unit.

Lateral Spreads

(…Contd)

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In earth spreads, the upper stable layer extends along a weaker underlying unit that has flowed following liquefaction or plastic deformation.

If the weaker unit is relatively thick, the overriding fractured blocks may subside into it, translate, rotate, disintegrate, liquefy, or even flow.

Lateral Spreads

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The spreads move slowly with moderate speed.

Their movement may sometimes be rapid after certain triggering mechanisms, such as an earthquake are activated.

The ground may then slowly spread over time from a few millimetres per day to tens of square meters per day

Velocity and triggers of spreads

(…Contd)

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The Triggers for spreads may be the presence of a weak layer.

Liquefaction of lower weak layer by earthquake shaking is a major trigger.

Natural or anthropogenic overloading of the ground above an unstable slope is yet another reason.


(…Contd)

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Saturation of underlying weaker layer due to precipitation, snowmelt, and ground-water changes may also induce spreads.

Plastic deformation of unstable material at depth (for example, salt) is another reason.


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Spreads can cause extensive property damage to buildings, roads, railroads, and lifelines.

They can spread slowly or quickly, depending on the extent of water saturation of the various soil layers.

Lateral spreads may be a precursor to earthflows.

Effects (direct/indirect) of spreads

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Preparation of liquefaction-potential maps is necessary.

Areas with potentially liquefiable soils can be avoided as construction sites, particularly in regions that are known to experience frequent earthquakes.

If high ground-water levels are involved, sites can be drained or other water-diversion efforts can be added.

Mitigation measures

(…Contd)

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Spreads may have high probability of recurring in areas that have experienced previous problems.

Most prevalent in areas that have an extreme earthquake hazard as well as liquefiable soils.

Lateral spreads are also associated with susceptible marine clays.

Mitigation measures

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Flooding Flooding

Landslides are much related to the influence of water.

Oversaturation of soil mass with water on the slope areas is a primary cause of landslides.

This effect can occur in the form of intense rainfall, snowmelt, changes in ground-water levels, and water-level changes along coastlines, earth dams, and the banks of lakes, reservoirs, canals, and rivers.

Overall triggers of landslides

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Flooding Flooding

Landsliding and flooding are closely allied because both are related to precipitation, runoff, and the saturation of ground by water.

In addition, debris flows and mudflows usually occur in small, steep stream channels and often are mistaken for floods; in fact, these two events often occur simultaneously in the same area.

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Landslides are also related to Seismic Activity.

Many mountainous areas that are vulnerable to landslides have also experienced at least moderate rates of earthquake occurrence in recorded times.

Seismic activity Seismic activity

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Landslides are also related to Volcanic Activity.

Landslides due to volcanic activity are some of the most devastating types.

Volcanic lava may melt snow at a rapid rate, causing a deluge of rock, soil, ash, and water that accelerates rapidly on the steep slopes of volcanoes, devastating anything in its path.

Volcanic activityVolcanic activity

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Landslides affect manmade structures whether they are directly on or near a landslide. Residential dwellings built on unstable slopes may experience partial damage to complete destruction as landslides destabilize or destroy foundations, walls, surrounding property, and above-ground and underground utilities.

One of the greatest potential consequences from landslides is to the transportation industry, and this commonly affects a large number of people all around the world.

Effects of Landslides on the Built Environment

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Landslides have effects on the natural environment. They affect the morphology of the Earth’s surface—mountain and valley systems, both on the continents and beneath the oceans.

The mountain and valley morphologies are most significantly affected by downslope movement of large landslide masses.

Effects of Landslides on the Natural Environment

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The forests and grasslands that cover much of the continents and the native wildlife that exists on the Earth’s surface and in its rivers, lakes, and seas, will be severely affected.

Forest, grasslands, and wildlife often are negatively affected by landslides.

The forest and fish habitats are most easily damaged.

Effects of Landslides on the Natural Environment

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How to Reduce the Effects of Landslides is a major step in disaster management.

Vulnerability to landslide hazards is a function of location, type of human activity, use and frequency of landslide events.

Landslide Mitigation methods

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The effects of landslides on people and structures can be lessened by total avoidance of landslide hazard areas or by restricting, prohibiting, or imposing conditions on hazard-zone activity.

The hazard from landslides can be reduced by avoiding construction on steep slopes and existing landslides, or by stabilizing the slopes.

Landslide Mitigation methods

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Technological Tools for Evaluation of Landslides

Mapping, Remote Sensing, and Monitoring are the major techniques for analysing and evaluating the landslides.

One of the guiding principles of geology is that the present is the key to the past and past will help to know about the future.

(…Contd)

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Technological Tools for Evaluation of Landslides

In evaluating the landslide hazards, the future slope failures could occur as a result of the same geologic, geomorphic, and hydrologic situations that led to the past and are leading the present failures.

Based on this assumption, it is possible to estimate the types, frequency of occurrence, extent, and consequences of slope failures that may occur in the future.

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Human-induced conditions, such as changes in the natural topography or hydrologic conditions, can create or increase an area’s susceptibility to slope failure.

Useful conclusions concerning increased probability of landsliding can be drawn by combining geological analyses with knowledge of short- and long-term meteorological conditions.

Monitoring is a must

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Map analysis is usually one of the first steps in a landslide investigation.

Necessary maps include bedrock and surficial geology, topography, soils, and if available, geomorphology maps.

Using knowledge of geologic materials and processes, a trained person can obtain a general idea of landslide susceptibility from such maps.

Map Analysis

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Analysis of aerial photography is a quick and valuable technique for identifying landslides, because it provides a three-dimensional overview of the terrain and indicates human activities as well as much geologic information to a trained person.

In addition, the availability of many types of aerial imagery or satellite images makes aerial reconnaissance very versatile and cost-effective.

Aerial surveys

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Many of the more subtle signs of slope movement cannot be identified on maps or aerial photographs.

Indeed, if an area is heavily forested or if it has been fully urbanized, even major features may not be evident. Furthermore, landslide features change over time on an active slide.

Field Reconnaissance Field Reconnaissance

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Field reconnaissance is always mandatory to verify or detect landslide features.

It is also needed to critically evaluate the potential instability of vulnerable slopes.

It identifies the areas of past landslides by using field mapping and laboratory testing of terrain through the sampling of soil and rock.

Field Reconnaissance Field Reconnaissance

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Geophysical techniques are suitable to determine some subsurface characteristics such as the depth to bedrock, stratigraphic layers, zones of saturation, and sometimes the ground-water table.

They are also useful to determine texture, porosity, and degree of consolidation of subsurface materials and the geometry of the units involved.

Geophysical Studies Geophysical Studies

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ConclusionConclusion

In India, landslides occur frequently in the regions of Himalayas, Western Ghats, Eastern Ghats and other hill ranges like Vindhyas.

The vulnerability to landslide hazards is a function of a site’s location (topography, geology, drainage), type of activity, and frequency of past landslides.

(…Contd)

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The effects of landslides on people and structures can be lessened by total avoidance of landslide hazard areas or by restricting, prohibiting, or imposing conditions on hazard-zone activity.

The Disaster Management is an emerging subject of earth science.

We have to minimize the impacts of natural hazards, like landslides.

(…Contd)

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Our ultimate aim is to protect the life and all properties.

For proper understanding of their impacts and causative factors, the landslide hazard zonation maps are to be prepared.

The knowledge of geology of the region is essential for mitigating the effects of landslides.

The impacts of landslides can me minimized through several mitigation methods that are available.

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